The present invention relates to a vacuum forming method and apparatus in which a thermoplastic sheet material to be formed is pushed, by at least one plug, into a recess provided in a female mold.
Various kinds of vacuum-forming methods are conventionally known from, for example, Japanese Patent Laid-Open Application No. 59-192529, in which a thermoplastic sheet material is drawn under vacuum so as to be brought into intimate contact with a wall surface of a recess formed in a female mold, to thereby form the sheet material. In recent years, a plug-assist forming method has largely been utilized for the advantageous reason that deep-drawing is possible, and in view of other advantages. FIGS. 1(A) through 1(C) of the accompanying drawings illustrate an example of a vacuum-forming apparatus for carrying the plug-assist forming method into effect. In this connection, reference should be made to "Plastic Kako Gijutsu Binran (Plastic Processing Technical Handbook)" written by Kenji Okubo and published by Nikkan Kogyo Shimbun-sha on July 10, 1984, pp. 435-443. As illustrated in FIG. 1(A), a female mold 1 has formed therein a recess 1a which opens to an upper surface of the female mold 1. The recess 1a is adapted to communicate with evacuation means, not shown, through communication bores 1b and a pipe 2. The reference numeral 3 designates a plug for pushing a thermoplastic sheet material 4 into the recess 1a. The plug 3 is substantially analogous in configuration to the recess 1a.
In forming the sheet material 4 by the vacuum forming apparatus constructed as described above, the sheet material 4 is first arranged above the female mold 1 and is heated by a heater, not shown, to soften the sheet material 4. The plug 3 is then moved to a predetermined position within the recess la to push the sheet material 4 thereinto, as shown in FIG. 1(B). Subsequently, a substantially closed space defined by the recess 1a and the sheet material is evacuated to bring the sheet material 4 into intimate contact with the wall surface of the recess 1a, as shown in FIG. 1(C). The sheet material 4 is then cooled and cured.
It is possible for the above-described plug-assist forming method to draw the sheet material deeper than a straight forming method in which a sheet material is merely drawn under vacuum so as to be brought into intimate contact with a wall surface of a recess without use of a plug. However, the plug-assist forming method has a certain limit in the deep-drawing, and cannot be said sufficiently satisfactory.
Specifically, as indicated by the two-dot-and-dash lines in FIG. 1(A), as the plug 3 moves toward the recess 1a, the plug 3 is brought into contact with the sheet material 4 to urge the same, and brings the sheet material 4 into contact with the female mold 1. As a result, the sheet material 4 is restrained at a point A by the female mold 1 and at a point B by the plug 3. As the plug 3 further moves downwardly, an extremely narrow portion of the sheet material 4 between the points A and B is largely stretched. Thus, a portion of a formed product corresponding to the portion of the sheet material 4 between the points A and B becomes thinner than the remaining portion. For this reason, it is not possible for the conventional plug-assist forming method to obtain a sufficiently satisfied drawing depth. Furthermore, if the peripheral wall surface of the recess 1a extends vertically, a gap between the peripheral wall surface of the recess 1a and the peripheral surface of the plug 3 cannot help but be narrowed inevitably, so that the above-mentioned problem becomes more and more marked. Thus, it becomes difficult per se to carry the plug-assist forming method into effect.